This invention relates to electron multipliers, and more particularly, to an electron multiplier having a conversion dynode that is biased by the same power supply that biases the detector component of the electron multiplier, through use of a DC-to-DC voltage converter.
A continuous dynode electron multiplier ("CDEM") is used in various scientific instruments, such as mass spectrometers. The CDEM senses the presence of one or more atomic or subatomic particles at its input, such as an ion or an electron, and provides a plurality of electrons at its output. The output electrons provide an indication of a characteristic of the primary radiation input to the electron multiplier. The output electrons are generated by secondary emissions through repeated collisions of initially the input particle, and then secondary electrons, with the inner surface of the detector component of the CDEM.
More specifically, the input particle(s) enters the detector portion of the CDEM and is accelerated therethrough by an electrostatic field of usually several thousand volts applied across the length of the detector. The inner surface of the detector is typically comprised of glass that is heavily lead-doped. The outer supporting body of the detector is usually stainless steel or other suitable material. Each time the particle strikes the inner surface of the detector, one or more secondary electrons are generated which themselves travel down the detector. A plurality of secondary electrons will be released from the glass if the electrons striking the inner surface of the detector have accumulated enough energy from the applied electrostatic field. Depending upon the design of the CDEM, this process of secondary emission is repeated numerous times throughout the entire length of the detector. Total electron multiplication (i.e., gain) of the CDEM from input particle detection through output electron emission is typically 10.sup.4 -10.sup.8.
The use of a conversion dynode as a separate element in the electron multiplier is well known. The conversion dynode improves the high mass sensitivity of the overall detector when used in scientific instruments such as mass spectrometers. That is, the conversion dynode gives the electron multiplier increased detection capability of particles of relatively greater mass. In such a configuration, the electron multiplier comprises the detector and the conversion dynode, which is separately biased apart from the detector. The operating voltage (i e., "bias") supplied to the detector is in the range of 800 v-3 kv, while the operating voltage of the conversion dynode is 4-20 kv.
Because of the difference in operating voltages, it is known in the art to provide two separate power supplies located external to the CDEM. Also, because of the separate supplies, two separate high voltage feedthroughs are also required. The feedthroughs accommodate the electrical connections from the power supplies through the package enclosure of the CDEM and into the internal portion of the CDEM held at vacuum pressure, the internal CDEM portion being where the detector and conversion dynode are located.
However, drawbacks exist in such a dual power supply configuration. An initial consideration is the added cost of the separate supply needed for the conversion dynode. A greater problem, however, may lie in the second feedthrough. Feedthroughs are relatively expensive to implement in an electron multiplier. They can also cause undesirable electrical noises to be generated if not properly designed and maintained. Further, sometimes existing mass spectrometers initially designed without a conversion dynode are desired to be upgraded to include a conversion dynode. To accommodate the conversion dynode, typically an additional power supply external to the electron multiplier package must be provided, if possible. Heretofore, no known retrofit of an existing mass spectrometer to include an electron multiplier having a conversion dynode has been possible without adding an additional power supply within the mass spectrometer yet external to the electron multiplier package, and without adding the corresponding additional feedthrough in the electron multiplier package.
Additionally, it has been discovered in those configurations where separate supplies are provided for the conversion dynode and detector that damage to the instrument (e.g., the mass spectrometer) may result in instances of sudden vacuum leaks. In these situations, the instrument turns off the detector power supply, while the supply for the conversion dynode is either not turned off or is turned off much too slowly, resulting in damage to the instrument.
Accordingly, it is a primary object of the present invention to bias both the detector and the conversion dynode elements of a CDEM using a single power supply external to the CDEM and a single corresponding feedthrough into the CDEM package.
It is a general object of the present invention to eliminate the separate power supply and feedthrough used for a conversion dynode required in a prior art electron multiplier having a conversion dynode.
It is a further object of the present invention to provide separate and different bias voltages for the detector and conversion dynode elements, respectively, of an electron multiplier by utilizing a single power supply and feedthrough along with a DC-to-DC voltage converter.
It is a still further object of the present invention to allow a scientific instrument such as a mass spectrometer to have its present electron multiplier without a conversion dynode replaced with an electron multiplier with a conversion dynode without having to modify by adding or changing any of the existing circuitry or components in the mass spectrometer external to the electron multiplier package; more specifically, without having to add an additional power supply to bias the conversion dynode, and without having to provide a corresponding separate feedthrough therefor in the electron multiplier package.
It is a still further object of the present invention to prevent damage to the scientific instrument incorporating the electron multiplier having both a detector and a conversion dynode by having the single supply that powers both the detector and the conversion dynode turn off in the case of vacuum leaks in the electron multiplier package.
It is yet another object of the present invention to incorporate a DC-to-DC voltage converter into the electron multiplier package without forcing a redesign of the package; the DC-to-DC voltage converter being connected to the existing detector power supply located external to the electron multiplier package; the DC-to-DC voltage converter not causing an overload of current drain on the external detector power supply.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.